1. Field of the Invention
The present invention relates to a light emitting device comprising a plurality of pixels each including a light emitting element and a means for supplying current to the light emitting element.
2. Description of the Related Art
Since a light emitting element emits light by itself, it is highly visible and does not require a back light which is needed in a liquid crystal display device (LCD). Therefore, it is suitably applied to thin devices and not restricted in viewing angle. Because of these advantages, a light emitting device having a light emitting element has recently been drawing attentions as an alternative display device to a CRT and an LCD. It is to be noted that a light emitting element in this specification indicates an element whose luminance is controlled by current or voltage, and it includes an OLED (Organic Light Emitting Diode) or an MIM electron source element (electron discharge element) and the like which is used in an FED (Field Emission Display).
Also, a light emitting device of the invention includes a panel and a module obtained by mounting an IC or the like onto the panel. More generally, the invention relates to an element substrate which corresponds to the condition before the completion of a panel in manufacturing steps of the light emitting device, and the element substrate comprises a plurality of pixels each having a unit for supplying current to a light emitting element.
OLED which is one of the light emitting elements includes an anode layer, a cathode layer, and a layer containing an electroluminescent material (hereinafter referred to as an electroluminescent layer) that generates luminescence (electroluminescence) when an electric field is applied thereto. The electroluminescent layer is provided between the anode and cathode, and it comprises a single or multiple layers. These layers may contain an inorganic compound. The electroluminescence in the electroluminescent layer includes a light emission (fluorescence) when a singlet exciting state returns to a ground state and a light emission (phosphorescence) when a triplet exciting state returns to a ground state.
The pixel configuration of a general light emitting device and its drive are described now in brief. A pixel shown in FIG. 7 comprises a switching transistor 700, an erasing transistor 708, a driving transistor 701, a capacitor 702, and a light emitting element 703. The gate of the switching transistor 700 is connected to a first scan line 705. Either the source or drain of the switching transistor 700 is connected to a signal line 704, and the other is connected to the gate of the driving transistor 701. The source of the driving transistor 701 is connected to a power supply line 706, and the drain thereof is connected to the anode of the light emitting element 703. The gate of the erasing transistor 708 is connected to a second scan line 709, the source thereof is connected to the power supply line 706, and the drain thereof is connected to the gate of the driving transistor 701. The cathode of the light emitting element 703 is connected to a counter electrode 707. The capacitor 702 is provided for storing a potential difference between the gate and source of the driving transistor 701. Also, the predetermined voltages are each applied to the power supply line 706 and the counter electrode 707 from a power supply and each has a potential difference.
When the switching transistor 700 is turned ON by a signal from the first scan line 705, a video signal that is input to the signal line 704 is input to the gate of the driving transistor 701. The potential difference between a potential of the input video signal and that of the power supply line 706 corresponds to a gate-source voltage Vgs of the driving transistor 701. Thus, current is supplied to the light emitting element 703, and the light emitting element 703 emits light by using the supplied current.